The present invention relates to audio signal compression.
International Patent Application WO 98/16014 discloses a data compression apparatus for data compressing an audio signal. The data compression apparatus comprises an input terminal for receiving the audio signal, a 1-bit A/D converter for A/D converting the audio signal so as to obtain a bitstream signal, a lossless coder for carrying out a lossless data compression step on the bit-stream signal so as to obtain a data compressed bit-stream signal, and an output terminal for supplying the data compressed bit-stream signal. Further, a recording apparatus and a transmitter apparatus comprising the data compression apparatus are disclosed. In addition, a data expansion apparatus for data expanding the data compressed bit-stream signal supplied by the data compression apparatus is disclosed, as well as a reproducing apparatus and a receiver apparatus comprising the data expansion apparatus.
It is an object of the invention to provide advantageous compression gain estimation in audio compressing. To this end the invention provides a method and device for estimating a compression gain, a recording apparatus and a transmitter as defined in the independent claims. Advantageous embodiments are defined in the dependent claims.
A compression gain estimation can be used in authoring and/or editing of audio signal sequences such as music pieces. It may be used for control of signal processing parameters, such as adaptive control of the compression ratio by compressive encoding, to allow recording of the content of e.g. a music piece within the limited maximum storage content of a recording medium such as a Super Audio Compact Disc (SACD) by providing a quick estimate of the amount of data that can be stored or recorded on the storage or recording medium.
Theoretically, to produce such an estimate it could be envisaged to use the average actual compression ratio or coding gain for a small fraction of an audio signal sequence such as a piece of music as an indication of the compression ratio for the entire sequence. Such an approach would suffer, however, from the problem that typical audio signal sequences in the form of music pieces have a rather wide variation of compression ratios or coding gains with significant short-time correlations, whereby a very significant fraction of the signal sequence would have to be used for obtaining a compression ratio estimate having the required level of precision or accuracy. Due to the required time for computation such a solution would in practice not be acceptable.
According to a first aspect of the invention an advantageous estimate of the obtainable compression gain is obtained by extraction of a signal power contained in a selected frequency band of the audio signal and obtaining said estimate by correlation with the extracted signal power. This aspect of the invention is based on the insight that a quick estimate of the compression ratio or coding gain can be obtained by using a correlation between the signal power of an audio signal and the compression gain. This aspect is especially advantageous in losslessly compressing 1 bit bit-stream signals e.g. in Direct Stream Digital format.
It is noted that from standard PCM coding it is known that the attainable coding gain is directly related to the degree of structure (flatness) in the signal power. However, in the above aspect of the invention the signal power itself is used, not its structure.